1. Technical Field
The present invention relates to a honeycomb structure, a honeycomb structure assembly, and a honeycomb catalyst.
This application claims benefit of priority to Japanese Patent Application No.2004-028186, filed on Feb. 4, 2004, the contents Of which are incorporated by reference herein.
2. Prior Art to be Disclosed
A honeycomb catalyst including a honeycomb structure with a catalyst component carried thereon is used to convert the exhaust gas of vehicles. The honeycomb catalyst is conventionally manufactured by making a material of a high specific surface area, for example, active alumina, and a catalyst metal, for example, platinum, carried on the surface of an integral cordierite honeycomb structure of low thermal expansion characteristic. An alkaline earth metal, such as Ba, is also carried on the honeycomb structure as a NOx storage agent to store NOx in an excess oxygen atmosphere of, for example, lean burn engines and Diesel engines. The enhanced conversion performance is attained by raising the potential for bringing the exhaust gas into contact with the noble metal catalyst and the NOx storage agent. For this purpose, it is required to increase the specific surface area of the carrier while decreasing the particle size of the noble metal and making the particles of the noble metal highly dispersed. A simple increase in quantity of the material having the high specific surface area, for example, active alumina, may, however, only increase the thickness of an alumina layer, while not significantly increasing the contact potential and undesirably heightening a pressure loss. One proposed technique accordingly specifies the shape of cells, the cell density, and the wall thickness to enhance the contact potential (see, for example, Japanese Patent Laid-Open Publication No. 10-263416). Another known honeycomb structure is manufactured by extrusion molding a material of a high specific surface area with inorganic fibers and an inorganic binder (see, for example, Japanese Patent Laid-Open Publication No. 5-213681). Still another proposed technique regulates the thickness of each wall between adjoining through holes (hereafter referred to as the wall thickness), the length in a gas flow direction, the length of each through hole, and the aperture ratio to preset values in an extrusion-molded honeycomb structure made of a material having a high specific surface area and a catalyst component carried thereon (see, for example, Japanese Patent Laid-Open Publication No. 2003-245547).
The prior art honeycomb structures cited above, however, have some drawbacks. The cordierite honeycomb structure disclosed in Japanese Patent Laid-Open Publication No. 10-263416 has the material having the high specific surface area, for example, active alumina, and the catalyst component, for example, platinum, carried on the surface thereof. This prior art technique specifies the shape of cells, the cell density, and the wall thickness to increase the specific surface area of the catalyst carrier and thereby enhance the potential for bringing the exhaust gas into contact with the catalyst component. This prior art technique, however, has the problem of a pressure loss and thereby does not sufficiently increase the specific surface area of the catalyst carrier. The insufficient specific surface area of the catalyst carrier results in insufficient dispersion of the catalyst component and poor conversion performance of the exhaust gas after heat aging. A significant increase in quantity of the catalyst component and size growth of the catalyst carrier may compensate for such insufficiency. Platinum and other noble metals are, however, very expensive and moreover limited precious resources. The size increase of the catalyst carrier is not desirable when the honeycomb structure with the catalyst is mounted on a limited space, such as an automobile.
The honeycomb structure disclosed in Japanese Patent Laid-Open Publication No. 5-213681 is obtained by extrusion molding the material having a high specific surface area with the inorganic fibers and the inorganic binder. This gives a catalyst carrier of a high specific surface area and attains sufficiently high dispersion of a catalyst component. The simple increase in specific surface area of the catalyst carrier may, however, fail to increase the potential for bringing the exhaust gas into contact with the catalyst component. This prior art technique may thus not attain efficient conversion of the exhaust gas.
The technique disclosed in Japanese Patent Laid-Open Publication No. 2003-245547 gives some suggestions about the relation between the catalytic performance and some parameters of the honeycomb structure, which include the wall thickness, the length in the gas flow direction, the length of each through hole, and the aperture ratio of the honeycomb structure. Simple regulation of these parameters may, however, fail to efficiently convert the exhaust gas.
The contents of Japanese Patent Laid-Open Publication No. 10-263416, Japanese Patent Laid-Open Publication No. 5-213681, and Japanese Patent Laid-Open Publication No. 2003-245547 are incorporated herein by reference in their entity.